Chapter 7 constraints and triggers
Download
1 / 41

Chapter 7 Constraints and Triggers - PowerPoint PPT Presentation


  • 127 Views
  • Uploaded on

Chapter 7 Constraints and Triggers . Spring 2011 Instructor: Hassan Khosravi. SQL: Constraints and Triggers. Certain properties we’d like our database to hold Modification of the database may break these properties Data entry may have errors Build handlers into the database definition.

loader
I am the owner, or an agent authorized to act on behalf of the owner, of the copyrighted work described.
capcha
Download Presentation

PowerPoint Slideshow about ' Chapter 7 Constraints and Triggers ' - patch


An Image/Link below is provided (as is) to download presentation

Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author.While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server.


- - - - - - - - - - - - - - - - - - - - - - - - - - E N D - - - - - - - - - - - - - - - - - - - - - - - - - -
Presentation Transcript
Chapter 7 constraints and triggers

Chapter 7 Constraints and Triggers

Spring 2011

Instructor: Hassan Khosravi


Sql constraints and triggers
SQL: Constraints and Triggers

  • Certain properties we’d like our database to hold

  • Modification of the database may break these properties

  • Data entry may have errors

  • Build handlers into the database definition


Sql constraints and triggers1
SQL: Constraints and Triggers

  • (integrity) constraints (static)

    • Constrain allowable database states

  • Triggers (Dynamic)

    • Monitor database changes

    • Check conditions and initiate conditions


Integrity constraints
(integrity) Constraints

  • Impose restrictions on allowable data beyond those imposed by structure and types

    • Examples on university database

      • 0 < gpa < 4.0

      • Enrollment < 50,000

      • Decision attribute: ‘y’ or ‘n’

      • Major = ‘CS’  decision = null

      • sizeHS < 200  not addmittedenr > 5000

  • Why use integrity constraints

    • Data-entry error (insert)

      • gpa in range

    • Correctness criteria (update)

    • Enforce consistency

      • Referenced tuples

    • Tell system about your data


Classification of integrity constraints
Classification of Integrity Constraints

  • Non- null

  • Keys

    • Uniqueness

  • Referential integrities (foreign key)

  • Attribute-based

    • Constraining values in attributes

  • Tuples-based

    • How values in different tuples should correlate

  • General assertions


Declaration and enforcing constraints
Declaration and enforcing Constraints

  • Declaration

    • With original schema when tables are declared

    • Once you have a running database

  • Enforcement

    • Check after every dangerous modification

      • Changing major we don’t need to check the gpa constraint

    • Deferred constraint checking

      • We may do some modifications that would raise errors

      • But after we have done all the modifications it should be ok

      • Check once some modifications are done (transaction)


Triggers
Triggers

  • Event-Condition-Action rules

    • When event occurs, check condition, if true, then do action

    • Example

      • Enrolllment > 75000  reject all applications

      • If application with gpa > 3.95  accept automatically

      • Update sizehs to be > 7000  change to wrong and raise error

  • Why use triggers?

    • Move codes from application to DBMS

    • Enforce constraints

      • Some of the assertions and checks are not implemented in some DBMS

      • Triggers could not only detect the problem, they can also solve it


Non null constraints
Non-null constraints

  • Defining that a specific attribute in a specific table can not take the value of null

    • Create table A(A1int, A2 int not null, A3 text)

    • Examples


Key constraints
Key Constraints

  • The primary key of the tables has to be unique

    • Create table A (A1int primary key, A2 int, A3 text)

    • Create table A (A1int, A2 int, A3 text) primary key A1, A2))

  • You can also define other combination of attributes to be unique ( without declaring them as key)

    • Create table A (A1int primary key, A2 int, A3 text unique)

    • Create table A (A1int primary key, A2 int, A3 text , unique(A1, A2))

  • Examples


Attribute base check constraints
Attribute base check constraints

  • Constraints on the attributes to have specific ranges or types

    • Create table A (A1int, A2 int, A3 text, check (A2 >0))

  • Examples


Assertions
Assertions

  • Assertions are very strong they are checks done over all tables at the same time. (assertions are generally not implemented in current dbms)

    • Create assertion name check ()

  • Example


Referential integrity
Referential integrity

  • Referential integrity is integrity of references.

    • No dangling pointers

    • What would referential integrity from S.sid to A.sid mean?











Triggers1
Triggers

  • Event-Condition-Action rules

    • When event occurs, check condition, if true, then do action

    • Example

      • Enrolllment > 75000  reject all applications

      • If application with gpa > 3.95  accept automatically

      • Update sizehs to be > 7000  change to wrong and raise error





Tricky issues with triggers
Tricky issues with Triggers

  • Row-Level vs. Statement-level

    • Which one do you use?

  • Multiple triggers activated at the same time

    • For example a delete command triggers multiple triggers

    • Which one do you run first?

  • Chaining trigger action

    • Self triggering

    • Cycles Trigger A Trigger B Trigger C Trigger A

    • Nested triggers

  • Implementations are significantly different in different systems

    • Tricky example


Trigger examples
Trigger Examples

  • For each student with GPA between 3.3 and 3.6, make them apply to geology in Stanford and biology in MIT


Trigger examples1
Trigger Examples

  • Implement referential integrity using triggers.

    • Delete all applications of a student from apply table after the student is deleted in the student table


Trigger examples2
Trigger Examples

  • Implement referential integrity using triggers on cascade.

    • Update the name of all colleges in the apply table if the name of the college is updated in the college table


Trigger examples3
Trigger Examples

  • Implement primary key using triggers.

    • Before inserting into table college check if the college exists. Raise an error if you get a duplicate

    • Example


Trigger examples4
Trigger Examples

  • Once a college has received 10 applications, add “-Done” to the end of the name of the college

  • Example of chains in triggers


Trigger examples5
Trigger Examples

  • If the size of the sizehk <100 or sizehk> 5000, then assume that its an error and ignore

  • Example considering before and after insert in chaining


Trigger examples6
Trigger Examples

  • Automatically accept students applying to Berkeley that have GPA > 3.7 and are coming from a highschool > 1200


Trigger examples7
Trigger Examples

  • Once a college passes a thresh-hold of 16000 applicants, delete new EE applicant and set all the decisions for all majors to undecided


More trigger examples self triggers
More Trigger Examples (self triggers)

  • By default this may get triggered only once.

  • Recursive_trigger = on


More trigger examples cycles
More Trigger Examples (cycles)

Example


More trigger examples conflicts
More Trigger Examples (Conflicts)

  • What happens when you have multiple triggers being triggered at the same time


More trigger examples nested triggers
More Trigger Examples (Nested Triggers)

  • What order are triggers carried out in you have nested triggers

    • behaves like imperative programming



ad